Particle-in-cell simulations on parametric instability of the lower hybrid wave

Abstract

Lower hybrid (LH) wave is an effective tool for current drive (CD) on tokamak devices. Parametric instability (PI) has always been a troubling phenomenon decreasing the lower hybrid current drive efficiency. In this work, the PI behavior of the LH waves for plasma parameters on the Experimental Advanced Superconducting Tokamak is investigated via first-principle simulation with a two-dimensional full-particle-in-cell method. The PI processes where an LH pump wave decays into another LH sideband and a low-frequency mode [ion sound quasi-mode (ISQM) or ion cyclotron quasi-mode (ICQM)] are observed in simulations. The ICQM-type and ISQM-type decay channels dominate, respectively, in the high- and low-plasma regimes while appearing simultaneously in the intermediate regime, which agrees well with the theoretical prediction by solving the parametric dispersion relation. In addition, for both the decay channels, their LH sidebands are excited with a wide range of parallel refractive index N∥ up to 30.0, which can resonantly interact with thermal electrons. The resulting electron heating can be enhanced due to overlap of resonances between the electrons and sidebands. Such electron heating not only dissipates the wave energy in the edge plasma but produces a significant portion of fast electrons, which may seriously affect tokamak discharges. The ion cyclotron heating due to the ICQMs is also observed in simulations.